Methods and apparatus for controlling a continuous flow rotary blood pump

ABSTRACT

A control system for a continuous flow rotary blood pump is provided. A normal operating range of the blood pump is established. The normal operating range may comprise a normal pump flow range and a normal pressure head range. A target rotational speed of the pump is set in accordance with the normal operating range. A current operating condition of the blood pump is determined. The current operating condition may comprise a current pump flow, a current pressure head, and a current rotational speed of the pump. The current operating condition is compared with the normal operating range. An appropriate control algorithm is selected from a plurality of available control algorithms based on the comparison. The target rotational speed of the pump is adjusted using the selected control algorithm to maintain or recover the normal operating range.

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 10/241,825 filed on Sep. 10, 2002 now U.S. Pat. No.6,817,836.

BACKGROUND OF THE INVENTION

The present invention relates to the field of rotary blood pumps forassisting a failing human heart. More specifically, the presentinvention relates to the control of continuous flow rotary blood pumpwhich does not compete with the bodies autonomic system unless the pumpflow is detected to be outside of a predetermined range.

Various types of rotary blood pumps have been developed and arecurrently under development for use as heart assist devices. Compared topulsatile pumps, rotary blood pumps have several advantages, includingsmaller size, higher efficiency, and a simpler design.

However, a servo control system for such rotary blood pumps has yet tobe developed. Typically, operators have had to monitor patients in theintensive care unit in order to observe the condition of the pump andthe patient, as manual intervention is currently required forcontrolling the rotational speed of the pump.

If such a rotary blood pump is to be used as a left ventricular assistdevice (LVAS), the pump flow should be increased when the pressureheadis decreased with the fixed rotational speed of the pump, because theseparameters automatically adjust to the patient's physiologicalcondition, regardless of the fixed rotational speed of the pumpimpeller. However, when the venous return suddenly becomes too lowbecause of physiological changes or overpumping, a high negativepressure may result at the inlet port of the pump, which may lead to asuction condition within the atrium and veins, which condition mayresult in serious injury or even death to the patient.

As rotary blood pumps may be used outside of a hospital environment, forexample in nursing homes and home health care environments, it would beadvantageous to provide an automated control system for controlling therotary blood pump, without the need for human supervision andintervention. It would be advantageous for such a control system tooperate the rotary blood pump automatically and effectively in responseto any sudden changes in the operating conditions of the pump whichdeviate from a normal operating range.

The methods and apparatus of the present invention provide the foregoingand other advantages.

SUMMARY OF THE INVENTION

The present invention relates to a control system for a continuous flowrotary blood pump, such as a centrifugal pump or an axial flow pump.

In an example embodiment, a normal operating range of the blood pump isestablished. The normal operating range may comprise a normal pump flowrange and a normal pressure head range. A target rotational speed of thepump can then be set in accordance with the normal operating range. Acurrent operating condition of the blood pump is determined. The currentoperating condition may comprise a current pump flow, a current pressurehead, and a current rotational speed of the pump. The current operatingcondition can then be compared with the normal operating range. Anappropriate control algorithm is then selected from a plurality ofavailable control algorithms based on the comparison. The targetrotational speed of the pump can then be adjusted using the selectedcontrol algorithm to maintain or recover the normal operating range.

The rotary blood pump may be used as a left ventricular assist device ora right ventricular assist device. The blood pump may be an implantabledevice or an external device.

Measurements of the current pump flow, the current pressure head, andthe current rotational speed may be used to determine the currentoperating condition. The current pump flow, the current pressure head,and the current rotational speed may be measured by one or more sensors.Such sensors may be implantable sensors. Alternatively, such sensors maybe external sensors.

The normal operating range may be established by determining a targetoperating point for the target rotational speed of the pump, whichprovides a target pump flow and a target pressure head. The normal pumpflow range may be within a 20% deviation from the target pump flow. Thenormal pressure head range may be within a 25% deviation from the targetpressure head.

During the normal operating range a normal operating condition controlalgorithm is selected. In such a case, the target rotational speed ofthe pump is maintained by applying proportional and derivative gaincontrol to the pump. The proportional and derivative gain control may beapplied in accordance with the formula:u=K _(p)(Y _(target) −Y)+K _(d)((d/dt)Y _(target)−(d/dt)Y)where u is a driving signal of the pump; Y is the rotational speed ofthe pump; Y_(target) is the target rotational speed of the pump; K_(p)is the proportional gain; and K_(d) is the derivative gain. As anexample, K_(p) may be set to approximately 0.02 and K_(d) may be set toapproximately 0.05.

In the event that the current operating condition is above the normaloperating range, a first abnormal operating condition control algorithmis selected. This algorithm decreases the target rotational speed untilthe normal operating range is recovered. For example, the targetrotational speed may be decremented by x rpm every t seconds until thenormal operating range is recovered. For a centrifugal pump, researchhas shown that the normal operating range may be recovered by, forexample, decrementing the rotational speed by approximately 150 rpmevery 5 seconds. For an axial flow pump, research has shown that thenormal operating range may be recovered by, for example, decrementingthe rotational speed by approximately 600 rpm every 5 seconds.

In the event that the current operating condition is below the normaloperating range, a second abnormal operating condition control algorithmis selected. This algorithm increases the target rotational speed untilthe normal operating range is recovered. For example, the targetrotational speed may be incremented by x rpm every t seconds until thenormal operating range is recovered. For a centrifugal pump, researchhas shown that the normal operating range may be recovered by, forexample, incrementing the rotational speed by approximately 150 rpmevery 5 seconds. For an axial flow pump, research has shown that thenormal operating range may be recovered by, for example, incrementingthe rotational speed by approximately 600 rpm every 5 seconds.

However, when the normal operating range cannot be recovered byincrementing the rotational speed of the pump, is determined that asuction condition exists (e.g., due to overpumping). In the event ofsuch a suction condition, a suction condition control algorithm isselected. This algorithm causes the suction condition to be released bycontinuously decreasing the target rotational speed of the pump toobtain a pump flow free from suction and free from overpumping. Once thesuction condition is released, the target rotational speed of the pumpis gradually increased to recover the normal operating range.

When releasing the suction condition, the target rotational speed may becontinuously decremented by x₁ rpm every t seconds. Once the suctioncondition is released, the target rotational speed may be continuouslyincremented by x₂ rpm every t seconds to recover the normal operatingcondition.

For a centrifugal pump, research has shown that the suction conditionmay be released when the rotational speed of the pump is decremented byapproximately 150 rpm every 5 seconds (e.g., x₁ is approximately 150 rpmand t is approximately 5). The normal operating range can then berecovered by incrementing the rotational speed of the pump byapproximately 50 rpm every 5 seconds (e.g., x₂ is approximately 50 rpmand t is approximately 5 seconds).

For an axial flow pump, research has shown that the suction conditionmay be released when the rotational speed of the pump is decremented byapproximately 600 rpm every 5 seconds (e.g., x₁ is approximately 600 rpmand t is approximately 5). The normal operating range can then berecovered by incrementing the rotational speed of the pump byapproximately 200 rpm every 5 seconds (e.g., x₂ is approximately 200 rpmand t is approximately 5 seconds).

In an example embodiment of the invention, the pump may comprise acentrifugal pump having magnets implanted in the pump impeller. One ormore Hall sensors may be used to detect the position of the pumpimpeller using the well-known Hall effect. The Hall sensors may be usedto detect vertical and/or horizontal movement of the pump impeller. Therpm of the impeller may be adjusted based on the position as detected bythe Hall sensors in order to maintain the impeller position in one of atop contact position or a dynamic suspension position. For example,where the Hall sensors detect the impeller position is at or near abottom contact position, it is desirable to increase the impeller rpm sothat the impeller position is moved towards a dynamic suspensionposition or a top contact position, as blood clots may form if theimpeller remains in a bottom contact position.

In a further example embodiment of the invention, the rotary blood pumpmay comprise an axial flow pump. The axial flow pump may have one ormore magnets implanted in the impeller. When the pump is used as aventricular assist device, the impeller may be affected by the pulsationof the natural heart. For example, the impeller may be moved back andforth horizontally along the axis of the impeller suspension system. Itis desirable to maintain the impeller in a position of dynamicsuspension between the supporting structures in order prevent theformation of blood clots or thrombi. A gap between the impeller and thesupporting structures of at least 100 microns is necessary to preventthe formation of thrombi. Such an anti-thrombogenic position (dynamicsuspension position) may be equated with a normal operating condition ofthe pump.

Corresponding methods and apparatus are provided for controlling thecontinuous flow rotary blood pump in accordance with the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe appended drawing figures, wherein like numerals denote likeelements, and:

FIG. 1 shows a block diagram of an example embodiment of the invention;

FIG. 2 is a graphical illustration of operating conditions of an exampleimplementation of the invention;

FIG. 3 shows a flowchart of an example embodiment of the invention;

FIG. 4 shows an example embodiment of a centrifugal pump with a Hallsensor in accordance with the invention;

FIG. 5 shows an example Hall sensor configuration for use with theinvention;

FIG. 6 shows a graph of the impeller position versus rpm of the pump ofFIG. 4;

FIG. 7 shows an example model circulation loop used in testing of anembodiment of the invention;

FIG. 8 is a graphical illustration of the convergence response time ofan a continuous flow blood pump in accordance with an example embodimentof the invention;

FIG. 9 (FIGS. 9( i)-9(iv)) show experimental results for a modelimplementation of the invention when recovering from an abnormalcondition;

FIG. 10 (FIGS. 10( i)-10(iv)) show experimental results for a modelimplementation of the invention when recovering from a suctioncondition; and

FIG. 11 shows an example embodiment of an axial flow pump in accordancewith the invention.

DETAILED DESCRIPTION OF THE INVENTION

The ensuing detailed description provides preferred exemplaryembodiments only, and is not intended to limit the scope, applicability,or configuration of the invention. Rather, the ensuing detaileddescription of the preferred exemplary embodiments will provide thoseskilled in the art with an enabling description for implementing apreferred embodiment of the invention. It should be understood thatvarious changes may be made in the function and arrangement of elementswithout departing from the spirit and scope of the invention as setforth in the appended claims.

The present invention provides control means for a rotary blood pump. Inorder to operate the pump as a durable anti-thrombogenic rotary bloodpump, the pump impeller must be suspended dynamically by eitherhydraulic or magnetic means when implanted as a bypass pump for afailing heart. The rotary blood pump may comprise, for example, acentrifugal pump or an axial flow pump.

For a centrifugal blood pump, dynamic suspension of the impeller isestablished when the impeller RPM is synchronized with the beatingnatural heart. Within a certain RPM range, the impeller achieves dynamicsuspension. Typically, for a centrifugal blood pump, the impeller RPMrange should be approximately 2,000±1,000 RPM and produce 4 L/min±2L/min pump flow. The impeller moves toward the top of the pump duringthe diastolic phase of the natural heart; whereas, during the systolicphase of the natural heart, the impeller moves toward the bottom of thepump. To maintain the dynamic suspension, a specified gap between themale and female bearings is required. Typically, the gap should measuremore than 400μ. If this gap is less than 400μ (typically, down to 80μ),effective suspension of the impeller to prevent blood clot formationdoes not occur. As a result of the impeller movement between the top andbottom housings, stagnant blood areas inside the pump are reduced oreliminated cyclically (typically at around 100 beat/min). Thus,thrombogenic areas inside the blood pump are also eliminated.

In order to further accelerate the reduction of thrombogenic areasinside a blood pump whose inflow tube is not vertical or whose radius ofthe male (smaller) and female (larger) bearings are different, theswinging motion of the impeller must be generated and synchronized withthe natural heart. Typically, this swinging motion of the impeller wouldbe generated when the impeller is at the top contact mode inside of theblood pump.

Dynamic suspension of the impeller of an axial flow blood pump is alsopossible under proper conditions. Dynamic suspension of the impellershould be synchronized with the beating natural heart so that theimpeller moves back and forth horizontally along an axis of the impellersuspension system. Such impeller movements should prevent blood clotformations at the gap between the impeller and its forward-and-backwardsupporting structure. The gap required for this type of dynamicsuspension should be greater than approximately 100μ. As indicated for acentrifugal pump, to eliminate blood clot formation inside an axial flowblood pump, it is imperative that the impeller movement be synchronizedwith the beating of the natural heart.

Typically, an axial flow blood pump would be operated within the rangeof 10,000±4,000 RPM in order to generate clinically needed blood flow of4 L/min±2 L/min. The dynamic suspension of the impeller should beestablished within this RPM range such that a gap of at least 100μbetween the impeller and the supporting structures is maintained duringthe back and forth movement of the impeller caused by the beating heart.A gap of greater than 100μ is preferred, but may vary depending on pumpstructure. For example, decoupling may occur due to too large of a gapfor the axial flow pump structure.

Suspension of the impeller may also be achieved magnetically. Regardlessof the means of suspension, the back and forth movement of the impelleralong an axis of the flow pump structure is required to achieveendurance and thrombus-free operation.

In an example embodiment as shown in FIG. 1, a rotary blood pump 10 isprovided which has an established normal operating range. The normaloperating range may comprise a normal pump flow range and a normalpressure head range. A motor driver 12 is provided to enable driving ofthe pump 10 at a target rotational speed in accordance with the normaloperating range. A condition estimator 14 is provided to enabledetermination of a current operating condition of the blood pump 10. Thecurrent operating condition may comprise a current pump flow, a currentpressure head, and a current rotational speed of the pump. A processor16 is provided to: (1) enable a comparison between the current operatingcondition and the normal operating range; and (2) enable the selectionof an appropriate control algorithm from a plurality of availablecontrol algorithms 18 based on the comparison. A controller 20 enablesadjustment of the target rotational speed of the pump using the selectedcontrol algorithm in order to maintain or recover the normal operatingrange.

The rotary blood pump 10 may be used as a left ventricular assist deviceor a right ventricular assist device. The rotary blood pump 10 may be animplantable device or an external device.

Measurements of the current pump flow and the current pressure head(shown collectively as 22) may be provided to the condition estimator 14to determine the current operating condition. A measurement of thecurrent rotational speed 24 may be provided to the controller 20 asfeedback for use in maintaining the target rotational speed of the pump10. The current pump flow, the current pressure head, and the currentrotational speed may be measured by one or more sensors. The one or moresensors may be implantable sensors. Those skilled in the art willappreciate that external sensors may also be used to implement theinvention.

The normal operating range may be established by determining a targetoperating point for the target rotational speed of the pump 10, whichprovides a target pump flow and a target pressure head. The normal pumpflow range may be set as a 20% deviation from the target pump flow. Thenormal pressure head range may be set as a 25% deviation from the targetpressure head.

FIG. 2 shows is a plot of pump flow versus pressurehead whichillustrates an example of a normal operating range centered about atarget operating point. The target operating point in the example shownin FIG. 2 is 100 mm Hg at 5 liters per minute. The normal pump flowrange is within a 20% deviation from the target operating point pumpflow or within the range of 4 to 6 liters per minute. The normalpressurehead is within a 25% deviation from the target operating pointpressurehead or within the range of 75 to 125 mm Hg. Outside of thenormal operating range an abnormal condition or a suction condition willbe detected.

The normal pump flow range and the normal pressure head range areestablished by taking into consideration the influences of the body'scircadian rhythm, physiological changes of the body, and measured noise.Within the normal operating range, it is assumed that the human body'snatural physiological control mechanisms will adjust the currentcondition by changing the afterload on the pump 10, such as the totalperipheral resistance. In this way, the inventive control mechanism doesnot interfere or compete with the body's autonomic system, unless anabnormal condition is detected. An abnormal condition is one in whichthe current pump flow or pressurehead are outside of the normaloperating range. Under such conditions, unusual problems may resultwhich are beyond the control of the body's physiological controlmechanisms.

During the normal operating range, the processor 16 selects a normaloperating condition control algorithm. The controller 20 maintains thetarget rotational speed of the pump 10 by applying proportional andderivative gain control to the pump 10, in accordance with the normaloperating condition control algorithm. The proportional and derivativegain control may be applied in accordance with the formula:u=K _(p)(Y _(target) −Y)+K _(d)((d/dt)Y _(target)−(d/dt)Y)where u is a driving signal of the pump 10; Y is the rotational speed ofthe pump 10; Y_(target) is the target rotational speed of the pump 10;K_(p) is the proportional gain; and K_(d) is the derivative gain. As anexample, K_(p) may be set to approximately 0.02 and K_(d) may be set toapproximately 0.05.

In the event that the current operating condition is above the normaloperating range, the processor 16 selects a first abnormal operatingcondition control algorithm. The controller decreases the targetrotational speed until the normal operating range is recovered, inaccordance with the first abnormal operating condition controlalgorithm. For example, the target rotational speed may be decrementedby x rpm every t seconds by the controller until the normal operatingrange is recovered. In an example embodiment where the pump is acentrifugal pump, x may be set to approximately 150 rpm and t may be setto approximately 5 seconds. In an example embodiment where the pump isan axial flow pump, x may be set to approximately 600 rpm and t may beset to approximately 5 seconds.

In the event that the current operating condition is below the normaloperating range, the processor 16 selects a second abnormal operatingcondition control algorithm. The controller 20 increases the targetrotational speed until the normal operating range is recovered, inaccordance with the second abnormal operating condition controlalgorithm. For example, the target rotational speed may be incrementedby x rpm every t seconds until the normal operating range is recovered.In an example embodiment where the pump is a centrifugal pump, x may beset to approximately 150 rpm and t may be set to approximately 5seconds. In an example embodiment where the pump is an axial flow pump,x may be set to approximately 600 rpm and t may be set to approximately5 seconds.

A suction condition may exist in the event that the normal operatingrange is not recovered by increasing the target rotational speed. Thesuction condition may be caused when attempting to recover from theabnormal condition where the current operating condition is below thenormal operating range. As described above, in such an abnormaloperating condition, the target rotational speed is increased in anattempt to recover the normal operating range. However, if the abnormalcondition remains after increasing the target rotational speed, asuction condition may have occurred as a result of overpumping, whichresults in the inlet port of the pump pulling on the walls of the heart.

In the event of such a suction condition, the processor 16 selects asuction condition control algorithm. The controller 20 releases thesuction condition by continuously decreasing the target rotational speedof the pump 10 to obtain a pump flow free from suction and free fromoverpumping. Once the suction condition is released, the controller 20gradually increases the target rotational speed of the pump to recoverthe normal operating range. As an example, when releasing the suctioncondition, the target rotational speed may be continuously decrementedby x₁ rpm every t seconds. Once the suction condition is released, thetarget rotational speed may be continuously incremented by x₂ rpm everyt seconds. In an example embodiment where the pump is a centrifugalpump, x₁ may be set to approximately 150 rpm, x₂ may be set toapproximately 50 rpm, and t may be set to approximately 5 seconds. In anexample embodiment where the pump is a centrifugal pump, x₁ may be setto approximately 600 rpm, x₂ may be set to approximately 200 rpm, and tmay be set to approximately 5 seconds.

FIG. 3 shows a flowchart of an example embodiment of the invention. Thecurrent operating condition 110 is detected, including current pump flowand current pressurehead. The current operating condition is comparedwith the normal operating range to determine if the current operatingcondition is acceptable 120. If the current operating condition isacceptable, the normal operating condition control algorithm is selected130, and the target rotational speed of the pump is maintained asdescribed above in connection with FIG. 1.

If the current operating condition is not acceptable, the current pumpflow is compared with the normal pump flow range to determine whetherthe current pump flow is acceptable 140. If the current pump flow isabove the normal pump flow range, the first abnormal operating conditioncontrol algorithm is selected 150, and the target rotational speed ofthe pump is decreased to recover the normal operating range, asdescribed above in connection with FIG. 1. If the current pump flow isbelow the normal pump flow range, the second abnormal operatingcondition algorithm is selected 160, and the target rotational speed ofthe pump is increased to recover the normal operating range, asdescribed above in connection with FIG. 1. If the current pump flow iswithin the normal pump flow range, the current pressurehead is thencompared to the normal pressurehead range to determine if the currentpressurehead is acceptable 170. If the current pressurehead is above thenormal pressurehead range, the first abnormal operating conditioncontrol algorithm is selected 150, and the target rotational speed isreduced. If the current pressurehead is below the normal pressureheadrange, the second abnormal condition control algorithm is selected 160,and the target rotational speed is increased.

Once the second abnormal condition control algorithm is selected 160, itis determined whether a suction condition exists by determining thedifferentiated pump flow 180, if the differentiated pump flow is greaterthan zero, no suction condition exists. If the differentiated pump flowis less than zero, a suction condition exists and the suction conditioncontrol algorithm is selected 190, and the target rotational speed ofthe pump if first reduced to release the suction condition and thenincreased to recover the normal operating range as discussed above inconnection with FIG. 1.

As shown in FIG. 4, the rotary blood pump may be a centrifugal pump 10′.In an example embodiment, the centrifugal pump 10′ may have one or moremagnets 13 implanted in the impeller 15. When the pump 10′ is used as aventricular assist device, the impeller 15 may be affected by thepulsation of the natural heart. For example, during the diastolic phaseof the heart, the impeller 15 may be moved toward a top contact positionand in the systolic phase of the heart, the impeller 15 may be movedtoward a bottom contact position. The beating of the natural heart mayalso cause the impeller 15 to move from side to side. It is desirable tomaintain the impeller 15 in either a top contact position or in aposition of dynamic suspension in order prevent the formation of bloodclots or thrombi. Such an anti-thrombogenic position (top contactposition or dynamic suspension position) may be equated with a normaloperating condition of the pump 10′.

A Hall sensor 11 may be used to detect the movement in the impeller 15via the position of the magnets 13 using the well-known Hall effect. TheHall sensor 11 may be positioned between the pump 10′ and the pumpactuator 17. As shown in FIG. 5, the Hall sensor 11 may comprisemultiple Hall sensors 11 a-11 f arranged in a circular plate 19 with ahole 21 in the center thereof for positioning between the actuator 17and the pump 10′ (as shown in FIG. 4). Each sensor 11 a-11 f haselectrical leads 23 which may be connected, for example, to thecontroller 20 of FIG. 1.

FIG. 5 shows six Hall sensors 11 a-11 f. Those skilled in the art willappreciate that any number of Hall sensors may be used to implement theinvention, depending on the accuracy of the readings required.

Using the Hall sensor 11, an abnormal pump condition may be detected.For example, when the impeller 15 moves towards the bottom contactposition (which position may result in the formation of blood clots) therpm of the impeller 15 may be adjusted accordingly to compensate forsuch movement and return the impeller to the anti-thrombogenic positionof dynamic suspension or top contact position. As shown in the graph ofFIG. 6, increasing the impeller rpm will move the impeller 15 from abottom contact position toward the top contact position (indicated bythe 1.6 mm point in FIG. 6). Thus, a Hall sensor 11 may be used as partof the control mechanism to maintain the impeller 15 in either the topcontact position or dynamic suspension in order to prevent the formationof blood clots.

FIG. 7 shows an example of a model left heart circulation loop used intesting of an embodiment of the invention. In the example shown, thecontinuous flow rotary blood pump 10 is used as a left ventricularassist device and is positioned between the apex of the heart (shown inthe model as a bag 28) and the aortic chamber 32. Total peripheralresistance of the heart is provided by a rotating clamping device 34positioned between the aortic chamber 32 and the left atrial chamber 36.The heart can be modeled as a pulsatile flow pump 38. A mitral valve 40and an aortic valve 42 are also provided in the model. Pressuretransducers 44 and 46 are provided at the distal position of the mitralvalve 40 and the distal position of the aortic valve 42, respectively.Pressure transducers 44 and 46 measure pressures between the mitralvalve 40 and aortic valve 42. A flow meter 48 measures the flow at theoutput side of the pump 10. Rotational speed of the pump is controlledby the motor driver 12 as discussed above in connection with FIG. 1.

In testing, the model circulation loop shown in FIG. 7 is filled withwater. The bag 28 has a volume of 30 ml and is attached to the inletport of the pump 10 in order to provide for the simulation of suction inthe heart apex. The pressurehead used in testing is the aortic pressureas measured by the pressure transducer 46. Three experiments wereperformed using a centrifugal pump as a left ventricular assist devicein the model of FIG. 7 to test the performance of the inventive system.

Experiment 1

Evaluation of the Controller for Operating Point Control.

In this experiment, the behavior of the convergence of the rotationalspeed of the pump is evaluated when the target rotational speed ispurposely changed up and down. The results of this experiment are shownin FIG. 8.

FIG. 8 shows a graph of the rotational speed versus time, whichillustrates an example convergence response time of the pump 10 to theproportional and derivative control applied by the normal operatingcondition algorithm. In the example shown in FIG. 8, the variation ofthe pump rotational speed is set at 150 rpm per 5 seconds. The targetrotational speed of the pump (shown in dashed lines) is shown as beingadjusted upwards at the 4 second mark by 150 rpm (i.e. from 1750 rpm to1900 rpm). In response to the proportional and derivative control, theactual rotational speed of the pump (shown as a solid line) requiresapproximately 5 seconds to respond and converge on the new targetrotational speed of 1900 rpm.

Experiment 2

Evaluation of the Controller for Recovering from Abnormal Condition.

In this experiment, the normal operating condition is changed to anabnormal state by increasing the afterload in the mock circulation loopin order to evaluate the behavior of the controller 20 in recoveringfrom an abnormal condition. The afterload is increased by increasing thetotal peripheral resistance provided by the clamping device 34.

FIGS. 9( i) through 9(iv) illustrate the results of this experiment. Asshown in FIG. 9( i), by increasing the total peripheral resistance(afterload), the pump flow was decreased. When the pump flow falls underthe acceptable range (e.g., 4 L/min as shown in FIG. 9( i)), an abnormalcondition is detected and the second abnormal operating conditionalgorithm is selected by the controller 20. The pressurehead is keptwithin an acceptable range as shown in FIG. 9( ii). FIG. 9( iii) showsthe operation of the second abnormal control algorithm in increasing thetarget rotational speed of the pump 10 in order to recover normal pumpflow. In FIG. 9( ii), the target rotational speed of the pump isincreased by 150 rpm every 5 seconds until the normal operatingcondition is recovered, as shown in FIG. 9( iv).

Experiment 3

Evaluation of Controller for Releasing Pump From a Suction Condition.

To evaluate the controller 20 in recovering from a suction condition,the position of the inlet port of the pump 10 is changed and theafterload is increased. The position of the inlet port was set near thewall of the bag 28.

FIGS. 10( i) through 10(iv) illustrate the results of this experiment.As shown in FIG. 10( i), by changing the location of the inlet port andincreasing the afterload, a suction condition is generated. Once thecurrent operating condition falls under the normal operating range, thesecond abnormal operating condition is chosen as discussed above inconnection with FIG. 1 and the target rotational speed of the pump isincreased. As shown in FIG. 10( ii), the pressurehead can be maintainedwithin the acceptable range even during the occurrence of the suctioncondition. FIG. 10( iii) shows the increase in the target rotationalspeed of the pump to recover a normal pump flow range.

However, if the pump flow remains in an abnormal condition, a suctioncondition will be detected. The suction condition control algorithm willthen be selected and the target rotational speed of the pump firstdecreased to release the suction condition and then increased as shownin FIG. 10( iii). However, since the operational state of the modelcirculation loop does not change (i.e. the positioning of the inlet portand the afterload which caused the suction condition remain constantabsent outside intervention), the normal operating condition of themodel loop will not be re-established, as shown in FIG. 10( iv).

As can be seen from FIGS. 10( i) through 10(iv), the controller is ableto release the suction condition, but the normal operating conditioncannot be re-established. The mock circulation loop will revert to thesuction condition, as the state of the mock circulation loop will notchange on its own in response to the controller. In an in vivo study, itwould be expected that the changes in position of the inlet port willproduce a sudden decrease of blood flow at a predetermined rotationalspeed. The inventive control system will be able to recover from asuction condition, due to the adaptive nature of the body's ownphysiological control mechanisms.

The experiments described above have shown that, in a model environment,the control system is effective at controlling a rotary blood pump inorder to maintain and restore normal operating conditions.

As shown in FIG. 11, the rotary blood pump may comprise an axial flowpump 200. In an example embodiment, the axial flow pump 200 may have oneor more magnets 202 implanted in the impeller 204. A motor stator 226 isprovided for driving the impeller 202.

When the pump 200 is used as a ventricular assist device, the impeller204 may be affected by the pulsation of the natural heart. For example,the impeller 204 may be moved back and forth horizontally along the axis220 of the impeller suspension system. In the case of mechanicalsuspension of the impeller 204 by bearings 228, a space 230 is requiredfor the bearing movement. It is also desirable to maintain the impeller204 in a position of dynamic suspension between the supportingstructures 222 in order prevent the formation of blood clots or thrombi.A gap 224 between the impeller and the supporting structures 222 of atleast 100 microns is necessary to prevent the formation of thrombi. Suchan anti-thrombogenic position (dynamic suspension position) may beequated with a normal operating condition of the pump 200.

As with the centrifugal pump described above in connection with FIGS. 4and 5, a Hall sensor 240 may also be used with the axial flow pump 200to detect the position of the impeller 204 using the well-known Halleffect.

Dynamic suspension of the impeller 202 may also be achieved magneticallyinstead of mechanically such that the gap 224 is maintained.

It should now be appreciated that the present invention providesadvantageous methods and apparatus for controlling a continuous flowrotary blood pump, such as a centrifugal pump or an axial flow pump.

Although the invention has been described in connection with variousillustrated embodiments, numerous modifications and adaptations may bemade thereto without departing from the spirit and scope of theinvention as set forth in the claims.

1. A method for controlling an axial flow rotary blood pump, comprising:establishing a normal operating range of the blood pump, said normaloperating range comprising a normal pump flow range and a normalpressure head range; setting a target rotational speed of said pump inaccordance with said normal operating range; determining a currentoperating condition of the blood pump, said current operating conditioncomprising a current pump flow, a current pressure head, and a currentrotational speed of said pump; comparing the current operating conditionwith the normal operating range; selecting an appropriate controlalgorithm from a plurality of available control algorithms based on saidcomparison; and adjusting the target rotational speed of said pump usingsaid selected control algorithm to maintain or recover said normaloperating range; wherein during said normal operating range: a normaloperating condition control algorithm is selected; and said targetrotational speed of said pump is maintained by applying proportional andderivative gain control to said pump.
 2. A method in accordance withclaim 1, wherein: establishing said normal operating range comprisesestablishing a target operating point for the target rotational speed ofthe pump which provides a target pump flow and a target pressure head;said normal pump flow range is within a 20% deviation from said targetpump flow; and said normal pressure head range is within a 25% deviationfrom said target pressure head.
 3. A method in accordance with claim 1,wherein: said rotary blood pump is used as one of a left ventricularassist device or a right ventricular assist device.
 4. A method inaccordance with claim 1, wherein: measurements of said current pumpflow, said current pressure head, and said current rotational speed areused to determine the current operating condition.
 5. A method inaccordance with claim 4, wherein said current pump flow, said currentpressure head, and said current rotational speed are measured by one ormore sensors.
 6. A method in accordance with claim 5, wherein said oneor more sensors are implantable sensors.
 7. A method in accordance withclaim 1, wherein said proportional and derivative gain control isapplied in accordance with the formula:u=K _(p)(Y _(target) −Y)+K _(d)((d/dt)Y _(target)−(d/dt)Y); u is adriving signal of said pump; Y is the rotational speed of the pump;Y_(target) is the target rotational speed of the pump; K_(p) is theproportional gain; and K_(d) is the derivative gain.
 8. A method inaccordance with claim 7, wherein: K_(p) is set to approximately 0.02;and K_(d) is set to approximately 0.05.
 9. A method in accordance withclaim 1, wherein said blood pump comprises one of an implantable pump oran external pump.
 10. A method in accordance with claim 1, wherein saidaxial flow pump has magnets implanted in a pump impeller.
 11. A methodin accordance with claim 10, further comprising: detecting a position ofsaid pump impeller using one or more Hall sensors.
 12. A method inaccordance with claim 11, wherein said one or more Hall sensors detectat least one of vertical or horizontal movement of the pump impeller.13. A method in accordance with claim 11, further comprising: adjustingthe rpm of said impeller based on said detecting step in order tomaintain the impeller position in a dynamic suspension position.
 14. Amethod in accordance with claim 13, further comprising maintaining a gapbetween the impeller and supporting structures for the impeller of atleast approximately 100 microns.
 15. A method in accordance with claim13, wherein said dynamic suspension is achieved either magnetically ormechanically.
 16. A method for controlling an axial flow rotary bloodpump, comprising: establishing a normal operating range of the bloodpump, said normal operating range comprising a normal pump flow rangeand a normal pressure head range; setting a target rotational speed ofsaid pump in accordance with said normal operating range; determining acurrent operating condition of the blood pump, said current operatingcondition comprising a current pump flow, a current pressure head, and acurrent rotational speed of said pump; comparing the current operatingcondition with the normal operating range; selecting an appropriatecontrol algorithm from a plurality of available control algorithms basedon said comparison; and adjusting the target rotational speed of saidpump using said selected control algorithm to maintain or recover saidnormal operating range; wherein in the event that the current operatingcondition is above said normal operating range: a first abnormaloperating condition control algorithm is selected; and the targetrotational speed is decreased until the normal operating range isrecovered.
 17. A method in accordance with claim 16, wherein the targetrotational speed is decremented by x rpm every t seconds until saidnormal operating range is recovered.
 18. A method in accordance withclaim 17, wherein: x is approximately 600 rpm; and t is approximately 5seconds.
 19. A method for controlling an axial flow rotary blood pump,comprising: establishing a normal operating range of the blood pump,said normal operating range comprising a normal pump flow range and anormal pressure head range; setting a target rotational speed of saidpump in accordance with said normal operating range; determining acurrent operating condition of the blood pump, said current operatingcondition comprising a current pump flow, a current pressure head, and acurrent rotational speed of said pump; comparing the current operatingcondition with the normal operating range; selecting an appropriatecontrol algorithm from a plurality of available control algorithms basedon said comparison; and adjusting the target rotational speed of saidpump using said selected control algorithm to maintain or recover saidnormal operating range; wherein in the event that the current operatingcondition is below said normal operating range: a second abnormaloperating condition control algorithm is selected; and the targetrotational speed is increased until the normal operating range isrecovered.
 20. A method in accordance with claim 19, wherein the targetrotational speed is incremented by x rpm every t seconds until saidnormal operating range is recovered.
 21. A method in accordance withclaim 20, wherein: x is approximately 600 rpm; and t is approximately 5seconds.
 22. A method in accordance with claim 19, wherein a suctioncondition exists in the event that the normal operating range is notrecovered by increasing said target rotational speed, and in the eventof such a suction condition; a suction condition control algorithm isselected; the suction condition is released by continuously decreasingthe target rotational speed of the pump to obtain a pump flow free fromsuction and free from overpumping; and once said suction condition isreleased, the target rotational speed of the pump is gradually increasedto recover said normal operating range.
 23. A method in accordance withclaim 22, wherein: when releasing the suction condition, the targetrotational speed is continuously decremented by x₁ rpm every t seconds;and once the suction condition is released, the target rotational speedis continuously incremented by x₂ rpm every t seconds.
 24. A method inaccordance with claim 23, wherein: x₁ is approximately 600 rpm; x₂ isapproximately 200 rpm; and t is approximately 5 seconds.
 25. Apparatusfor controlling an axial flow rotary blood pump, comprising: a rotaryblood pump having an established normal operating range, said normaloperating range comprising a normal pump flow range and a normalpressure head range; a motor driver to enable driving of said pump at atarget rotational speed in accordance with said normal operating range;a condition estimator to enable determination of a current operatingcondition of the blood pump, said current operating condition comprisinga current pump flow, a current pressure head, and a current rotationalspeed of said pump; a processor to: (1) enable a comparison between thecurrent operating condition and the normal operating range; and (2)enable the selection of an appropriate control algorithm from aplurality of available control algorithms based on said comparison; anda controller to enable adjustment of the target rotational speed of saidpump using said selected control algorithm to maintain or recover saidnormal operating range; wherein during said normal operating range: saidprocessor selects a normal operating condition control algorithm; andsaid controller maintains said target rotational speed of said pump byapplying proportional and derivative gain control to said pump. 26.Apparatus in accordance with claim 25, wherein: said normal operatingrange is established by determining a target operating point for thetarget rotational speed of the pump which provides a target pump flowand a target pressure head; said normal pump flow range is within a 20%deviation from said target pump flow; and said normal pressure headrange is within a 25% deviation from said target pressure head. 27.Apparatus in accordance with claim 25, wherein: said rotary blood pumpis used as one of a left ventricular assist device or a rightventricular assist device.
 28. Apparatus in accordance with claim 25,wherein: measurements of said current pump flow and said currentpressure head are provided to the condition estimator to determine thecurrent operating condition; and a measurement of said currentrotational speed is provided to said controller.
 29. Apparatus inaccordance with claim 28, wherein said current pump flow, said currentpressure head, and said current rotational speed are measured by one ormore sensors.
 30. Apparatus in accordance with claim 29, wherein saidone or more sensors are implantable sensors.
 31. Apparatus in accordancewith claim 25, wherein said proportional and derivative gain control isapplied in accordance with the formula:u=K _(p)(Y _(target) −Y)+K _(d)((d/dt)Y _(target)−(d/dt)Y); u is adriving signal of said pump; Y is the rotational speed of the pump;Y_(target) is the target rotational speed of the pump; K_(p) is theproportional gain; and K_(d) is the derivative gain.
 32. Apparatus inaccordance with claim 31, wherein: K_(p) is set to approximately 0.02;and K_(d) is set to approximately 0.05.
 33. Apparatus in accordance withclaim 25, wherein said blood pump comprises one of an implantable pumpor an external pump.
 34. Apparatus in accordance with claim 25, whereinsaid axial flow pump has magnets implanted in a pump impeller. 35.Apparatus in accordance with claim 34, further comprising: one or moreHall sensors for detecting a position of said pump impeller. 36.Apparatus in accordance with claim 35, wherein said one or more Hallsensors detect at least one of vertical or horizontal movement of thepump impeller.
 37. Apparatus in accordance with claim 35, wherein: therpm of the impeller is adjusted based on the impeller position asdetected by the one or more Hall sensors in order to maintain theimpeller position in a dynamic suspension position.
 38. Apparatus inaccordance with claim 37, wherein a gap is maintained between theimpeller and supporting structures for the impeller of at leastapproximately 100 microns.
 39. Apparatus in accordance with claim 37,wherein said dynamic suspension is achieved either magnetically ormechanically.
 40. Apparatus for controlling an axial flow rotary bloodpump, comprising: a rotary blood pump having an established normaloperating range, said normal operating range comprising a normal pumpflow range and a normal pressure head range; a motor driver to enabledriving of said pump at a target rotational speed in accordance withsaid normal operating range; a condition estimator to enabledetermination of a current operating condition of the blood pump, saidcurrent operating condition comprising a current pump flow, a currentpressure head, and a current rotational speed of said pump; a processorto: (1) enable a comparison between the current operating condition andthe normal operating range; and (2) enable the selection of anappropriate control algorithm from a plurality of available controlalgorithms based on said comparison; and a controller to enableadjustment of the target rotational speed of said pump using saidselected control algorithm to maintain or recover said normal operatingrange; wherein in the event that the current operating condition isabove said normal operating range: said processor selects a firstabnormal operating condition control algorithm; and said controllerdecreases the target rotational speed until the normal operating rangeis recovered.
 41. Apparatus in accordance with claim 40, wherein thetarget rotational speed is decremented by x rpm every t seconds untilsaid normal operating range is recovered.
 42. Apparatus in accordancewith claim 41, wherein: x is approximately 600 rpm; and t isapproximately 5 seconds.
 43. Apparatus for controlling an axial flowrotary blood pump, comprising: a rotary blood pump having an establishednormal operating range, said normal operating range comprising a normalpump flow range and a normal pressure head range; a motor driver toenable driving of said pump at a target rotational speed in accordancewith said normal operating range; a condition estimator to enabledetermination of a current operating condition of the blood pump, saidcurrent operating condition comprising a current pump flow, a currentpressure head, and a current rotational speed of said pump; a processorto: (1) enable a comparison between the current operating condition andthe normal operating range; and (2) enable the selection of anappropriate control algorithm from a plurality of available controlalgorithms based on said comparison; and a controller to enableadjustment of the target rotational speed of said pump using saidselected control algorithm to maintain or recover said normal operatingrange; wherein in the event that the current operating condition isbelow said normal operating range: said processor selects a secondabnormal operating condition control algorithm; and said controllerincreases the target rotational speed until the normal operating rangeis recovered.
 44. Apparatus in accordance with claim 43, wherein thetarget rotational speed is incremented by x rpm every t seconds untilsaid normal operating range is recovered.
 45. Apparatus in accordancewith claim 44, wherein: x is approximately 600 rpm; and t isapproximately 5 seconds.
 46. Apparatus in accordance with claim 43,wherein a suction condition exists in the event that the normaloperating range is not recovered by increasing said target rotationalspeed, and in the event of such a suction condition; said processorselects a suction condition control algorithm; said controller releasesthe suction condition by continuously decreasing the target rotationalspeed of the pump to obtain a pump flow free from suction and free fromoverpumping; and once said suction condition is released, saidcontroller gradually increases the target rotational speed of the pumpto recover said normal operating range.
 47. A method in accordance withclaim 46, wherein: when releasing the suction condition, the targetrotational speed is continuously decremented by x₁ rpm every t seconds;and once the suction condition is released, the target rotational speedis continuously incremented by x₂ rpm every t seconds.
 48. Apparatus inaccordance with claim 47, wherein: x₁ is approximately 600 rpm; x₂ isapproximately 200 rpm; and t is approximately 5 seconds.